A significant proportion (3-39%) of chronic psychiatric patients have been found to drink abnormal quantities of liquids (polydipsia). Among these patients, 25 to 86% have been found to suffer from hyponatremic symptoms (water intoxication) that can entail serious neurologic and psychiatric complications, and even death. Previous prevalence studies have lacked biological measures to verify polydipsia and water intoxication. Two biological measures (diurnal weight change and specific gravity of the urine) were used to establish the prevalence of these disorders in a state hospital sample (n=360). The prevalence of both disorders using these biological measures was higher than previously reported. Little is known about the natural course of polydipsia and water intoxication. All previous studies have measured these disorders at a single point in time. A 12-month longitudinal study is proposed involving four groups of patients distinguished by severity of polydipsia and the presence of water intoxication. A second state hospital sample (n=677), already screened for these disorders, will be utilized. The goals of this proposal are to: 1) use multiple biological and behavioral measures to establish criteria for water intoxication and polydipsia (accounting for differences related to gender and race), 2) estimate the prevalence of these disorders and measure their stability, and 3) determine the demographic and clinical characteristics associated with these disorders.